Hybrid work machines have conventionally been known in which an engine-assist motor generator is attached to an engine which drives a hydraulic pump, and a partially motorized driving mechanism is employed instead of a hydraulic driving mechanism, such as a hydraulic cylinder and a hydraulic motor. This type of hybrid work machine includes an electricity storage device to store generated power from the motor generator and regenerative power of an electric motor, and a power converter to convert these electric powers, a buck-boost converter and the like.
The motor generator is connected via the power converter to a DC bus, and the DC bus is connected via the buck-boost converter to the electricity storage device. The electric motor is connected via another power converter to the DC bus. When the motor generator and the electric motor perform power-run operation, the buck-boost converter boosts a voltage supplied by the electricity storage device to a DC input voltage to the power converter and the power converter converts DC to AC so as to drive the motor generator. When the electric motor performs regenerative operation, the regenerative power is converted to a DC voltage by the power converter, and is stepped down by the buck-boost converter and then stored in the electricity storage device.
In a hybrid work machine described in PTL 1, each of an engine-assist motor generator and an electric motor for turning is provided with a buck-boost converter and a DC bus from a common electricity storage device in order that different voltages are obtainable on these DC buses. That is, in the engine-assist motor generator to be used only at a constant rotation speed, the DC bus (an input DC voltage to a power converter) is set to an approximately constant voltage value. In the electric motor for driving turning whose output frequently changes according to a lever operation amount, a voltage value at the DC bus is allowed to change according to a rotation speed of the electric motor for turning, thereby achieving efficient driving.